A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, a radiation beam is used to project a desired pattern from the reticle onto the substrate. The dose of radiation that is received by the reticle causes heating of the reticle. This heating causes thermal expansion and as a result the shape of the reticle may change, therewith adversely affecting imaging performance, e.g. overlay and/or focus, of the lithographic process.
In a known embodiment of a lithographic apparatus, a finite elements method model (FEM model) is used to model the effects of heating of a reticle on imaging performance on a lithographic process. The output of this FEM model can be used to improve overlay and/or focus by adjusting positioning of the reticle with respect to the substrate during the actual projection of the patterned beam on the substrate.
With increasing demands on imaging performance of a lithographic apparatus, the complexity of the FEM model increases in order to effectively adjust the effects of reticle heating in imaging performance. Moreover, there is a need to cool the reticle with a reticle cooling system to cool the reticle below a certain temperature. The cooling provided by the reticle cooling system and its effect on reticle shape further increases the complexity of the FEM model.
The increasing complexity of the FEM model results in an increasing numerical effort to calculate the effect of reticle heating and/or cooling on overlay and/or focus. This may result in that the calculation of the effect of the reticle heating and/or cooling is not fast enough to use the result for adjustment of the positioning of the reticle and substrate with respect to each other for subsequent substrates.